Add Validation in Apex

public void beforeUpdate(List<Opportunity> newOpportunities, Map<Id, Opportunity> oldMap) {
    for (Opportunity opp : newOpportunities) {
        Opportunity oldOpp = oldMap.get(opp.Id);
        Boolean justWon = opp.StageName == 'Closed Won' && oldOpp.StageName != 'Closed Won';

        if (justWon && (opp.Amount == null || opp.Amount <= 0)) {
            opp.Amount.addError('An Opportunity cannot be marked Closed Won without a positive Amount.');
        }
    }
}

addError(), called on the specific field, blocks the entire save and shows the message directly next to that field — a much better user experience than a generic failure, and it prevents exactly the "Closed Won with a blank Amount silently rolls up as 0" scenario this lesson exists to close.

addError() only works meaningfully in a before trigger — calling it in an after trigger doesn't prevent the save, since the record is already saved by the time after runs (Module 24's before/after distinction, once again). This validation needs its own beforeUpdate method on the handler, separate from the rollup logic living in afterUpdate.

trigger OpportunityTrigger on Opportunity (before update, after update) {
    OpportunityTriggerHandler handler = new OpportunityTriggerHandler();

    if (Trigger.isBefore) {
        handler.beforeUpdate(Trigger.new, Trigger.oldMap);
    }
    if (Trigger.isAfter) {
        handler.afterUpdate(Trigger.new, Trigger.oldMap);
    }
}

This is Module 24's "Trigger Framework Fundamentals" routing pattern, now genuinely needed — one trigger, two events, each routed to its own clearly-named handler method.